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          Iterator 和 for-of 循环
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      </script></div> <div class="theme-vdoing-content content__default"><h1 id="iterator-和-for-of-循环"><a href="#iterator-和-for-of-循环" class="header-anchor">#</a> Iterator 和 for...of 循环</h1> <h2 id="iterator-遍历器-的概念"><a href="#iterator-遍历器-的概念" class="header-anchor">#</a> Iterator（遍历器）的概念</h2> <p>JavaScript 原有的表示“集合”的数据结构，主要是数组（<code>Array</code>）和对象（<code>Object</code>），ES6 又添加了<code>Map</code>和<code>Set</code>。这样就有了四种数据集合，用户还可以组合使用它们，定义自己的数据结构，比如数组的成员是<code>Map</code>，<code>Map</code>的成员是对象。这样就需要一种统一的接口机制，来处理所有不同的数据结构。

遍历器（Iterator）就是这样一种机制。<strong>它是一种接口，为各种不同的数据结构提供统一的访问机制</strong>。<strong>任何数据结构只要部署 Iterator 接口，就可以完成遍历操作</strong>（即依次处理该数据结构的所有成员）。</p> <p>Iterator 的作用有三个：一是为各种数据结构，提供一个统一的、简便的访问接口；二是使得数据结构的成员能够按某种次序排列；三是 ES6 创造了一种新的遍历命令<code>for...of</code>循环，Iterator 接口主要供<code>for...of</code>消费。</p> <p>Iterator 的遍历过程是这样的。</p> <p>（1）创建一个指针对象，指向当前数据结构的起始位置。也就是说，遍历器对象本质上，就是一个指针对象。</p> <p>（2）第一次调用指针对象的<code>next</code>方法，可以将指针指向数据结构的第一个成员。</p> <p>（3）第二次调用指针对象的<code>next</code>方法，指针就指向数据结构的第二个成员。</p> <p>（4）不断调用指针对象的<code>next</code>方法，直到它指向数据结构的结束位置。</p> <p>每一次调用<code>next</code>方法，都会返回数据结构的当前成员的信息。具体来说，就是返回一个包含<code>value</code>和<code>done</code>两个属性的对象。其中，<code>value</code>属性是当前成员的值，<code>done</code>属性是一个布尔值，表示遍历是否结束。</p> <p>下面是一个模拟<code>next</code>方法返回值的例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> it <span class="token operator">=</span> <span class="token function">makeIterator</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

it<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// { value: &quot;a&quot;, done: false }</span>
it<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// { value: &quot;b&quot;, done: false }</span>
it<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// { value: undefined, done: true }</span>

<span class="token keyword">function</span> <span class="token function">makeIterator</span><span class="token punctuation">(</span><span class="token parameter">array</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">var</span> nextIndex <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
  <span class="token keyword">return</span> <span class="token punctuation">{</span>
    <span class="token function-variable function">next</span><span class="token operator">:</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token keyword">return</span> nextIndex <span class="token operator">&lt;</span> array<span class="token punctuation">.</span>length <span class="token operator">?</span>
        <span class="token punctuation">{</span>value<span class="token operator">:</span> array<span class="token punctuation">[</span>nextIndex<span class="token operator">++</span><span class="token punctuation">]</span><span class="token punctuation">,</span> done<span class="token operator">:</span> <span class="token boolean">false</span><span class="token punctuation">}</span> <span class="token operator">:</span>
        <span class="token punctuation">{</span>value<span class="token operator">:</span> <span class="token keyword">undefined</span><span class="token punctuation">,</span> done<span class="token operator">:</span> <span class="token boolean">true</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
  <span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br></div></div><p>上面代码定义了一个<code>makeIterator</code>函数，它是一个遍历器生成函数，作用就是返回一个遍历器对象。对数组<code>['a', 'b']</code>执行这个函数，就会返回该数组的遍历器对象（即指针对象）<code>it</code>。</p> <p>指针对象的<code>next</code>方法，用来移动指针。开始时，指针指向数组的开始位置。然后，每次调用<code>next</code>方法，指针就会指向数组的下一个成员。第一次调用，指向<code>a</code>；第二次调用，指向<code>b</code>。</p> <p><code>next</code>方法返回一个对象，表示当前数据成员的信息。这个对象具有<code>value</code>和<code>done</code>两个属性，<code>value</code>属性返回当前位置的成员，<code>done</code>属性是一个布尔值，表示遍历是否结束，即是否还有必要再一次调用<code>next</code>方法。</p> <p>总之，调用指针对象的<code>next</code>方法，就可以遍历事先给定的数据结构。</p> <p>对于遍历器对象来说，<code>done: false</code>和<code>value: undefined</code>属性都是可以省略的，因此上面的<code>makeIterator</code>函数可以简写成下面的形式。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">makeIterator</span><span class="token punctuation">(</span><span class="token parameter">array</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">var</span> nextIndex <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
  <span class="token keyword">return</span> <span class="token punctuation">{</span>
    <span class="token function-variable function">next</span><span class="token operator">:</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token keyword">return</span> nextIndex <span class="token operator">&lt;</span> array<span class="token punctuation">.</span>length <span class="token operator">?</span>
        <span class="token punctuation">{</span>value<span class="token operator">:</span> array<span class="token punctuation">[</span>nextIndex<span class="token operator">++</span><span class="token punctuation">]</span><span class="token punctuation">}</span> <span class="token operator">:</span>
        <span class="token punctuation">{</span>done<span class="token operator">:</span> <span class="token boolean">true</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
  <span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>由于 Iterator 只是把接口规格加到数据结构之上，所以，遍历器与它所遍历的那个数据结构，实际上是分开的，完全可以写出没有对应数据结构的遍历器对象，或者说用遍历器对象模拟出数据结构。下面是一个无限运行的遍历器对象的例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> it <span class="token operator">=</span> <span class="token function">idMaker</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

it<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span>value <span class="token comment">// 0</span>
it<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span>value <span class="token comment">// 1</span>
it<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span>value <span class="token comment">// 2</span>
<span class="token comment">// ...</span>

<span class="token keyword">function</span> <span class="token function">idMaker</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">var</span> index <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>

  <span class="token keyword">return</span> <span class="token punctuation">{</span>
    <span class="token function-variable function">next</span><span class="token operator">:</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token keyword">return</span> <span class="token punctuation">{</span>value<span class="token operator">:</span> index<span class="token operator">++</span><span class="token punctuation">,</span> done<span class="token operator">:</span> <span class="token boolean">false</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
  <span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br></div></div><p>上面的例子中，遍历器生成函数<code>idMaker</code>，返回一个遍历器对象（即指针对象）。但是并没有对应的数据结构，或者说，遍历器对象自己描述了一个数据结构出来。</p> <p>如果使用 TypeScript 的写法，遍历器接口（Iterable）、指针对象（Iterator）和<code>next</code>方法返回值的规格可以描述如下。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">interface</span> <span class="token class-name">Iterable</span> <span class="token punctuation">{</span>
  <span class="token punctuation">[</span>Symbol<span class="token punctuation">.</span>iterator<span class="token punctuation">]</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">:</span> Iterator<span class="token punctuation">,</span>
<span class="token punctuation">}</span>

<span class="token keyword">interface</span> <span class="token class-name">Iterator</span> <span class="token punctuation">{</span>
  <span class="token function">next</span><span class="token punctuation">(</span>value<span class="token operator">?</span><span class="token operator">:</span> any<span class="token punctuation">)</span> <span class="token operator">:</span> IterationResult<span class="token punctuation">,</span>
<span class="token punctuation">}</span>

<span class="token keyword">interface</span> <span class="token class-name">IterationResult</span> <span class="token punctuation">{</span>
  value<span class="token operator">:</span> any<span class="token punctuation">,</span>
  done<span class="token operator">:</span> boolean<span class="token punctuation">,</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br></div></div><h2 id="默认-iterator-接口"><a href="#默认-iterator-接口" class="header-anchor">#</a> 默认 Iterator 接口</h2> <p>I<strong>terator 接口的目的，就是为所有数据结构，提供了一种统一的访问机制，即<code>for...of</code>循环（详见下文）</strong>。当使用<code>for...of</code>循环遍历某种数据结构时，该循环会自动去寻找 Iterator 接口。</p> <p><strong>一种数据结构只要部署了 Iterator 接口，我们就称这种数据结构是“可遍历的”</strong>（iterable）。</p> <p>ES6 规定，<strong>默认的 Iterator 接口部署在数据结构的<code>Symbol.iterator</code>属性</strong>，或者说，一个数据结构只要具有<code>Symbol.iterator</code>属性，就可以认为是“可遍历的”（iterable）。<code>Symbol.iterator</code>属性本身是一个函数，就是当前数据结构默认的遍历器生成函数。执行这个函数，就会返回一个遍历器。至于属性名<code>Symbol.iterator</code>，它是一个表达式，返回<code>Symbol</code>对象的<code>iterator</code>属性，这是一个预定义好的、类型为 Symbol 的特殊值，所以要放在方括号内（参见《Symbol》一章）。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span>
  <span class="token punctuation">[</span>Symbol<span class="token punctuation">.</span>iterator<span class="token punctuation">]</span> <span class="token operator">:</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token punctuation">{</span>
      <span class="token function-variable function">next</span><span class="token operator">:</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">return</span> <span class="token punctuation">{</span>
          value<span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">,</span>
          done<span class="token operator">:</span> <span class="token boolean">true</span>
        <span class="token punctuation">}</span><span class="token punctuation">;</span>
      <span class="token punctuation">}</span>
    <span class="token punctuation">}</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br></div></div><p>上面代码中，对象<code>obj</code>是可遍历的（iterable），因为具有<code>Symbol.iterator</code>属性。执行这个属性，会返回一个遍历器对象。该对象的根本特征就是具有<code>next</code>方法。每次调用<code>next</code>方法，都会返回一个代表当前成员的信息对象，具有<code>value</code>和<code>done</code>两个属性。</p> <p>ES6 的有些数据结构原生具备 Iterator 接口（比如数组），即不用任何处理，就可以被<code>for...of</code>循环遍历。原因在于，这些数据结构原生部署了<code>Symbol.iterator</code>属性（详见下文），另外一些数据结构没有（比如对象）。凡是部署了<code>Symbol.iterator</code>属性的数据结构，就称为部署了遍历器接口。调用这个接口，就会返回一个遍历器对象。</p> <p><strong>原生具备 Iterator 接口的数据结构如下。</strong></p> <ul><li>Array</li> <li>Map</li> <li>Set</li> <li>String</li> <li>TypedArray</li> <li>函数的 arguments 对象</li> <li>NodeList 对象</li></ul> <p>下面的例子是数组的<code>Symbol.iterator</code>属性。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> arr <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">,</span> <span class="token string">'c'</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> iter <span class="token operator">=</span> arr<span class="token punctuation">[</span>Symbol<span class="token punctuation">.</span>iterator<span class="token punctuation">]</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

iter<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// { value: 'a', done: false }</span>
iter<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// { value: 'b', done: false }</span>
iter<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// { value: 'c', done: false }</span>
iter<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// { value: undefined, done: true }</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码中，变量<code>arr</code>是一个数组，原生就具有遍历器接口，部署在<code>arr</code>的<code>Symbol.iterator</code>属性上面。所以，调用这个属性，就得到遍历器对象。</p> <p>对于原生部署 Iterator 接口的数据结构，不用自己写遍历器生成函数，<code>for...of</code>循环会自动遍历它们。除此之外，其他数据结构（主要是对象）的 Iterator 接口，都需要自己在<code>Symbol.iterator</code>属性上面部署，这样才会被<code>for...of</code>循环遍历。</p> <p>对象（Object）之所以没有默认部署 Iterator 接口，是因为对象的哪个属性先遍历，哪个属性后遍历是不确定的，需要开发者手动指定。本质上，遍历器是一种线性处理，对于任何非线性的数据结构，部署遍历器接口，就等于部署一种线性转换。不过，严格地说，对象部署遍历器接口并不是很必要，因为这时对象实际上被当作 Map 结构使用，ES5 没有 Map 结构，而 ES6 原生提供了。</p> <p>一个对象如果要具备可被<code>for...of</code>循环调用的 Iterator 接口，就必须在<code>Symbol.iterator</code>的属性上部署遍历器生成方法（原型链上的对象具有该方法也可）。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">RangeIterator</span> <span class="token punctuation">{</span>
  <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token parameter">start<span class="token punctuation">,</span> stop</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">this</span><span class="token punctuation">.</span>value <span class="token operator">=</span> start<span class="token punctuation">;</span>
    <span class="token keyword">this</span><span class="token punctuation">.</span>stop <span class="token operator">=</span> stop<span class="token punctuation">;</span>
  <span class="token punctuation">}</span>

  <span class="token punctuation">[</span>Symbol<span class="token punctuation">.</span>iterator<span class="token punctuation">]</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">;</span> <span class="token punctuation">}</span>

  <span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">var</span> value <span class="token operator">=</span> <span class="token keyword">this</span><span class="token punctuation">.</span>value<span class="token punctuation">;</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>value <span class="token operator">&lt;</span> <span class="token keyword">this</span><span class="token punctuation">.</span>stop<span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token keyword">this</span><span class="token punctuation">.</span>value<span class="token operator">++</span><span class="token punctuation">;</span>
      <span class="token keyword">return</span> <span class="token punctuation">{</span>done<span class="token operator">:</span> <span class="token boolean">false</span><span class="token punctuation">,</span> value<span class="token operator">:</span> value<span class="token punctuation">}</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
    <span class="token keyword">return</span> <span class="token punctuation">{</span>done<span class="token operator">:</span> <span class="token boolean">true</span><span class="token punctuation">,</span> value<span class="token operator">:</span> <span class="token keyword">undefined</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>

<span class="token keyword">function</span> <span class="token function">range</span><span class="token punctuation">(</span><span class="token parameter">start<span class="token punctuation">,</span> stop</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">return</span> <span class="token keyword">new</span> <span class="token class-name">RangeIterator</span><span class="token punctuation">(</span>start<span class="token punctuation">,</span> stop<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">var</span> value <span class="token keyword">of</span> <span class="token function">range</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 0, 1, 2</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br><span class="line-number">24</span><br><span class="line-number">25</span><br></div></div><p>上面代码是一个类部署 Iterator 接口的写法。<code>Symbol.iterator</code>属性对应一个函数，执行后返回当前对象的遍历器对象。</p> <p>下面是通过遍历器实现指针结构的例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">Obj</span><span class="token punctuation">(</span><span class="token parameter">value</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">this</span><span class="token punctuation">.</span>value <span class="token operator">=</span> value<span class="token punctuation">;</span>
  <span class="token keyword">this</span><span class="token punctuation">.</span>next <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token class-name">Obj</span><span class="token punctuation">.</span>prototype<span class="token punctuation">[</span>Symbol<span class="token punctuation">.</span>iterator<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">var</span> iterator <span class="token operator">=</span> <span class="token punctuation">{</span> next<span class="token operator">:</span> next <span class="token punctuation">}</span><span class="token punctuation">;</span>

  <span class="token keyword">var</span> current <span class="token operator">=</span> <span class="token keyword">this</span><span class="token punctuation">;</span>

  <span class="token keyword">function</span> <span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>current<span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token keyword">var</span> value <span class="token operator">=</span> current<span class="token punctuation">.</span>value<span class="token punctuation">;</span>
      current <span class="token operator">=</span> current<span class="token punctuation">.</span>next<span class="token punctuation">;</span>
      <span class="token keyword">return</span> <span class="token punctuation">{</span> done<span class="token operator">:</span> <span class="token boolean">false</span><span class="token punctuation">,</span> value<span class="token operator">:</span> value <span class="token punctuation">}</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
      <span class="token keyword">return</span> <span class="token punctuation">{</span> done<span class="token operator">:</span> <span class="token boolean">true</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
  <span class="token punctuation">}</span>
  <span class="token keyword">return</span> iterator<span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">var</span> one <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Obj</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">var</span> two <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Obj</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">var</span> three <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Obj</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

one<span class="token punctuation">.</span>next <span class="token operator">=</span> two<span class="token punctuation">;</span>
two<span class="token punctuation">.</span>next <span class="token operator">=</span> three<span class="token punctuation">;</span>

<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">var</span> i <span class="token keyword">of</span> one<span class="token punctuation">)</span><span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 1, 2, 3</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br><span class="line-number">24</span><br><span class="line-number">25</span><br><span class="line-number">26</span><br><span class="line-number">27</span><br><span class="line-number">28</span><br><span class="line-number">29</span><br><span class="line-number">30</span><br><span class="line-number">31</span><br><span class="line-number">32</span><br></div></div><p>上面代码首先在构造函数的原型链上部署<code>Symbol.iterator</code>方法，调用该方法会返回遍历器对象<code>iterator</code>，调用该对象的<code>next</code>方法，在返回一个值的同时，自动将内部指针移到下一个实例。</p> <p>下面是另一个为对象添加 Iterator 接口的例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span>
  data<span class="token operator">:</span> <span class="token punctuation">[</span> <span class="token string">'hello'</span><span class="token punctuation">,</span> <span class="token string">'world'</span> <span class="token punctuation">]</span><span class="token punctuation">,</span>
  <span class="token punctuation">[</span>Symbol<span class="token punctuation">.</span>iterator<span class="token punctuation">]</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">const</span> self <span class="token operator">=</span> <span class="token keyword">this</span><span class="token punctuation">;</span>
    <span class="token keyword">let</span> index <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
    <span class="token keyword">return</span> <span class="token punctuation">{</span>
      <span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">if</span> <span class="token punctuation">(</span>index <span class="token operator">&lt;</span> self<span class="token punctuation">.</span>data<span class="token punctuation">.</span>length<span class="token punctuation">)</span> <span class="token punctuation">{</span>
          <span class="token keyword">return</span> <span class="token punctuation">{</span>
            value<span class="token operator">:</span> self<span class="token punctuation">.</span>data<span class="token punctuation">[</span>index<span class="token operator">++</span><span class="token punctuation">]</span><span class="token punctuation">,</span>
            done<span class="token operator">:</span> <span class="token boolean">false</span>
          <span class="token punctuation">}</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
          <span class="token keyword">return</span> <span class="token punctuation">{</span> value<span class="token operator">:</span> <span class="token keyword">undefined</span><span class="token punctuation">,</span> done<span class="token operator">:</span> <span class="token boolean">true</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
      <span class="token punctuation">}</span>
    <span class="token punctuation">}</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br></div></div><p>对于类似数组的对象（存在数值键名和<code>length</code>属性），部署 Iterator 接口，有一个简便方法，就是<code>Symbol.iterator</code>方法直接引用数组的 Iterator 接口。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token class-name">NodeList</span><span class="token punctuation">.</span>prototype<span class="token punctuation">[</span>Symbol<span class="token punctuation">.</span>iterator<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token class-name">Array</span><span class="token punctuation">.</span>prototype<span class="token punctuation">[</span>Symbol<span class="token punctuation">.</span>iterator<span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token comment">// 或者</span>
<span class="token class-name">NodeList</span><span class="token punctuation">.</span>prototype<span class="token punctuation">[</span>Symbol<span class="token punctuation">.</span>iterator<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">[</span>Symbol<span class="token punctuation">.</span>iterator<span class="token punctuation">]</span><span class="token punctuation">;</span>

<span class="token punctuation">[</span><span class="token operator">...</span>document<span class="token punctuation">.</span><span class="token function">querySelectorAll</span><span class="token punctuation">(</span><span class="token string">'div'</span><span class="token punctuation">)</span><span class="token punctuation">]</span> <span class="token comment">// 可以执行了</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>NodeList 对象是类似数组的对象，本来就具有遍历接口，可以直接遍历。上面代码中，我们将它的遍历接口改成数组的<code>Symbol.iterator</code>属性，可以看到没有任何影响。</p> <p>下面是另一个<strong>类似数组的对象调用数组的<code>Symbol.iterator</code>方法的例子</strong>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> iterable <span class="token operator">=</span> <span class="token punctuation">{</span>
  <span class="token number">0</span><span class="token operator">:</span> <span class="token string">'a'</span><span class="token punctuation">,</span>
  <span class="token number">1</span><span class="token operator">:</span> <span class="token string">'b'</span><span class="token punctuation">,</span>
  <span class="token number">2</span><span class="token operator">:</span> <span class="token string">'c'</span><span class="token punctuation">,</span>
  length<span class="token operator">:</span> <span class="token number">3</span><span class="token punctuation">,</span>
  <span class="token punctuation">[</span>Symbol<span class="token punctuation">.</span>iterator<span class="token punctuation">]</span><span class="token operator">:</span> <span class="token class-name">Array</span><span class="token punctuation">.</span>prototype<span class="token punctuation">[</span>Symbol<span class="token punctuation">.</span>iterator<span class="token punctuation">]</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> item <span class="token keyword">of</span> iterable<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>item<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 'a', 'b', 'c'</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>注意，<strong>普通对象部署数组的<code>Symbol.iterator</code>方法，并无效果</strong>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> iterable <span class="token operator">=</span> <span class="token punctuation">{</span>
  a<span class="token operator">:</span> <span class="token string">'a'</span><span class="token punctuation">,</span>
  b<span class="token operator">:</span> <span class="token string">'b'</span><span class="token punctuation">,</span>
  c<span class="token operator">:</span> <span class="token string">'c'</span><span class="token punctuation">,</span>
  length<span class="token operator">:</span> <span class="token number">3</span><span class="token punctuation">,</span>
  <span class="token punctuation">[</span>Symbol<span class="token punctuation">.</span>iterator<span class="token punctuation">]</span><span class="token operator">:</span> <span class="token class-name">Array</span><span class="token punctuation">.</span>prototype<span class="token punctuation">[</span>Symbol<span class="token punctuation">.</span>iterator<span class="token punctuation">]</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> item <span class="token keyword">of</span> iterable<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>item<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// undefined, undefined, undefined</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>如果<code>Symbol.iterator</code>方法对应的不是遍历器生成函数（即会返回一个遍历器对象），解释引擎将会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>

obj<span class="token punctuation">[</span>Symbol<span class="token punctuation">.</span>iterator<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token number">1</span><span class="token punctuation">;</span>

<span class="token punctuation">[</span><span class="token operator">...</span>obj<span class="token punctuation">]</span> <span class="token comment">// TypeError: [] is not a function</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中，变量<code>obj</code>的<code>Symbol.iterator</code>方法对应的不是遍历器生成函数，因此报错。</p> <p>有了遍历器接口，数据结构就可以用<code>for...of</code>循环遍历（详见下文），也可以使用<code>while</code>循环遍历。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> $iterator <span class="token operator">=</span> <span class="token constant">ITERABLE</span><span class="token punctuation">[</span>Symbol<span class="token punctuation">.</span>iterator<span class="token punctuation">]</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">var</span> $result <span class="token operator">=</span> $iterator<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token operator">!</span>$result<span class="token punctuation">.</span>done<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">var</span> x <span class="token operator">=</span> $result<span class="token punctuation">.</span>value<span class="token punctuation">;</span>
  <span class="token comment">// ...</span>
  $result <span class="token operator">=</span> $iterator<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码中，<code>ITERABLE</code>代表某种可遍历的数据结构，<code>$iterator</code>是它的遍历器对象。遍历器对象每次移动指针（<code>next</code>方法），都检查一下返回值的<code>done</code>属性，如果遍历还没结束，就移动遍历器对象的指针到下一步（<code>next</code>方法），不断循环。</p> <h2 id="调用-iterator-接口的场合"><a href="#调用-iterator-接口的场合" class="header-anchor">#</a> 调用 Iterator 接口的场合</h2> <p>有一些场合会默认调用 Iterator 接口（即<code>Symbol.iterator</code>方法），除了下文会介绍的<code>for...of</code>循环，还有几个别的场合。</p> <p><strong>（1）解构赋值</strong></p> <p>对数组和 Set 结构进行解构赋值时，会默认调用<code>Symbol.iterator</code>方法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> set <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Set</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token string">'a'</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token string">'b'</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token string">'c'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token keyword">let</span> <span class="token punctuation">[</span>x<span class="token punctuation">,</span>y<span class="token punctuation">]</span> <span class="token operator">=</span> set<span class="token punctuation">;</span>
<span class="token comment">// x='a'; y='b'</span>

<span class="token keyword">let</span> <span class="token punctuation">[</span>first<span class="token punctuation">,</span> <span class="token operator">...</span>rest<span class="token punctuation">]</span> <span class="token operator">=</span> set<span class="token punctuation">;</span>
<span class="token comment">// first='a'; rest=['b','c'];</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p><strong>（2）扩展运算符</strong></p> <p>扩展运算符（...）也会调用默认的 Iterator 接口。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 例一</span>
<span class="token keyword">var</span> str <span class="token operator">=</span> <span class="token string">'hello'</span><span class="token punctuation">;</span>
<span class="token punctuation">[</span><span class="token operator">...</span>str<span class="token punctuation">]</span> <span class="token comment">//  ['h','e','l','l','o']</span>

<span class="token comment">// 例二</span>
<span class="token keyword">let</span> arr <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token string">'b'</span><span class="token punctuation">,</span> <span class="token string">'c'</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token operator">...</span>arr<span class="token punctuation">,</span> <span class="token string">'d'</span><span class="token punctuation">]</span>
<span class="token comment">// ['a', 'b', 'c', 'd']</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>上面代码的扩展运算符内部就调用 Iterator 接口。</p> <p>实际上，这提供了一种简便机制，可以将任何部署了 Iterator 接口的数据结构，转为数组。也就是说，<strong>只要某个数据结构部署了 Iterator 接口，就可以对它使用扩展运算符，将其转为数组</strong>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> arr <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token operator">...</span>iterable<span class="token punctuation">]</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p><strong>（3）yield*</strong></p> <p><code>yield*</code>后面跟的是一个可遍历的结构，它会调用该结构的遍历器接口。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> <span class="token function-variable function">generator</span> <span class="token operator">=</span> <span class="token keyword">function</span><span class="token operator">*</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">yield</span> <span class="token number">1</span><span class="token punctuation">;</span>
  <span class="token keyword">yield</span><span class="token operator">*</span> <span class="token punctuation">[</span><span class="token number">2</span><span class="token punctuation">,</span><span class="token number">3</span><span class="token punctuation">,</span><span class="token number">4</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
  <span class="token keyword">yield</span> <span class="token number">5</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token keyword">var</span> iterator <span class="token operator">=</span> <span class="token function">generator</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

iterator<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// { value: 1, done: false }</span>
iterator<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// { value: 2, done: false }</span>
iterator<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// { value: 3, done: false }</span>
iterator<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// { value: 4, done: false }</span>
iterator<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// { value: 5, done: false }</span>
iterator<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// { value: undefined, done: true }</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br></div></div><p><strong>（4）其他场合</strong></p> <p>由于数组的遍历会调用遍历器接口，所以任何接受数组作为参数的场合，其实都调用了遍历器接口。下面是一些例子。</p> <ul><li>for...of</li> <li>Array.from()</li> <li>Map(), Set(), WeakMap(), WeakSet()（比如<code>new Map([['a',1],['b',2]])</code>）</li> <li>Promise.all()</li> <li>Promise.race()</li></ul> <h2 id="字符串的-iterator-接口"><a href="#字符串的-iterator-接口" class="header-anchor">#</a> 字符串的 Iterator 接口</h2> <p><strong>字符串是一个类似数组的对象，也原生具有 Iterator 接口</strong>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> someString <span class="token operator">=</span> <span class="token string">&quot;hi&quot;</span><span class="token punctuation">;</span>
<span class="token keyword">typeof</span> someString<span class="token punctuation">[</span>Symbol<span class="token punctuation">.</span>iterator<span class="token punctuation">]</span>
<span class="token comment">// &quot;function&quot;</span>

<span class="token keyword">var</span> iterator <span class="token operator">=</span> someString<span class="token punctuation">[</span>Symbol<span class="token punctuation">.</span>iterator<span class="token punctuation">]</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

iterator<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span>  <span class="token comment">// { value: &quot;h&quot;, done: false }</span>
iterator<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span>  <span class="token comment">// { value: &quot;i&quot;, done: false }</span>
iterator<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span>  <span class="token comment">// { value: undefined, done: true }</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p>上面代码中，调用<code>Symbol.iterator</code>方法返回一个遍历器对象，在这个遍历器上可以调用 next 方法，实现对于字符串的遍历。</p> <p>可以覆盖原生的<code>Symbol.iterator</code>方法，达到修改遍历器行为的目的。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> str <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">String</span><span class="token punctuation">(</span><span class="token string">&quot;hi&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token punctuation">[</span><span class="token operator">...</span>str<span class="token punctuation">]</span> <span class="token comment">// [&quot;h&quot;, &quot;i&quot;]</span>

str<span class="token punctuation">[</span>Symbol<span class="token punctuation">.</span>iterator<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">return</span> <span class="token punctuation">{</span>
    <span class="token function-variable function">next</span><span class="token operator">:</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span>_first<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">this</span><span class="token punctuation">.</span>_first <span class="token operator">=</span> <span class="token boolean">false</span><span class="token punctuation">;</span>
        <span class="token keyword">return</span> <span class="token punctuation">{</span> value<span class="token operator">:</span> <span class="token string">&quot;bye&quot;</span><span class="token punctuation">,</span> done<span class="token operator">:</span> <span class="token boolean">false</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
      <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
        <span class="token keyword">return</span> <span class="token punctuation">{</span> done<span class="token operator">:</span> <span class="token boolean">true</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
      <span class="token punctuation">}</span>
    <span class="token punctuation">}</span><span class="token punctuation">,</span>
    _first<span class="token operator">:</span> <span class="token boolean">true</span>
  <span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token punctuation">[</span><span class="token operator">...</span>str<span class="token punctuation">]</span> <span class="token comment">// [&quot;bye&quot;]</span>
str <span class="token comment">// &quot;hi&quot;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br></div></div><p>上面代码中，字符串 str 的<code>Symbol.iterator</code>方法被修改了，所以扩展运算符（<code>...</code>）返回的值变成了<code>bye</code>，而字符串本身还是<code>hi</code>。</p> <h2 id="iterator-接口与-generator-函数"><a href="#iterator-接口与-generator-函数" class="header-anchor">#</a> Iterator 接口与 Generator 函数</h2> <p><code>Symbol.iterator</code>方法的最简单实现，还是使用下一章要介绍的 Generator 函数。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> myIterable <span class="token operator">=</span> <span class="token punctuation">{</span>
  <span class="token punctuation">[</span>Symbol<span class="token punctuation">.</span>iterator<span class="token punctuation">]</span><span class="token operator">:</span> <span class="token keyword">function</span><span class="token operator">*</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">yield</span> <span class="token number">1</span><span class="token punctuation">;</span>
    <span class="token keyword">yield</span> <span class="token number">2</span><span class="token punctuation">;</span>
    <span class="token keyword">yield</span> <span class="token number">3</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token punctuation">[</span><span class="token operator">...</span>myIterable<span class="token punctuation">]</span> <span class="token comment">// [1, 2, 3]</span>

<span class="token comment">// 或者采用下面的简洁写法</span>

<span class="token keyword">let</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span>
  <span class="token operator">*</span> <span class="token punctuation">[</span>Symbol<span class="token punctuation">.</span>iterator<span class="token punctuation">]</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">yield</span> <span class="token string">'hello'</span><span class="token punctuation">;</span>
    <span class="token keyword">yield</span> <span class="token string">'world'</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> x <span class="token keyword">of</span> obj<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">// &quot;hello&quot;</span>
<span class="token comment">// &quot;world&quot;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br></div></div><p>上面代码中，<code>Symbol.iterator</code>方法几乎不用部署任何代码，只要用 yield 命令给出每一步的返回值即可。</p> <h2 id="遍历器对象的-return-throw"><a href="#遍历器对象的-return-throw" class="header-anchor">#</a> 遍历器对象的 return()，throw()</h2> <p>遍历器对象除了具有<code>next</code>方法，还可以具有<code>return</code>方法和<code>throw</code>方法。如果你自己写遍历器对象生成函数，那么<code>next</code>方法是必须部署的，<code>return</code>方法和<code>throw</code>方法是否部署是可选的。</p> <p><code>return</code>方法的使用场合是，如果<code>for...of</code>循环提前退出（通常是因为出错，或者有<code>break</code>语句），就会调用<code>return</code>方法。如果一个对象在完成遍历前，需要清理或释放资源，就可以部署<code>return</code>方法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">readLinesSync</span><span class="token punctuation">(</span><span class="token parameter">file</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">return</span> <span class="token punctuation">{</span>
    <span class="token punctuation">[</span>Symbol<span class="token punctuation">.</span>iterator<span class="token punctuation">]</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token keyword">return</span> <span class="token punctuation">{</span>
        <span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
          <span class="token keyword">return</span> <span class="token punctuation">{</span> done<span class="token operator">:</span> <span class="token boolean">false</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span><span class="token punctuation">,</span>
        <span class="token keyword">return</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
          file<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
          <span class="token keyword">return</span> <span class="token punctuation">{</span> done<span class="token operator">:</span> <span class="token boolean">true</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
      <span class="token punctuation">}</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span><span class="token punctuation">,</span>
  <span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br></div></div><p>上面代码中，函数<code>readLinesSync</code>接受一个文件对象作为参数，返回一个遍历器对象，其中除了<code>next</code>方法，还部署了<code>return</code>方法。下面的两种情况，都会触发执行<code>return</code>方法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 情况一</span>
<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> line <span class="token keyword">of</span> <span class="token function">readLinesSync</span><span class="token punctuation">(</span>fileName<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>line<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token keyword">break</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token comment">// 情况二</span>
<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> line <span class="token keyword">of</span> <span class="token function">readLinesSync</span><span class="token punctuation">(</span>fileName<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>line<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">Error</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>上面代码中，情况一输出文件的第一行以后，就会执行<code>return</code>方法，关闭这个文件；情况二会在执行<code>return</code>方法关闭文件之后，再抛出错误。</p> <p>注意，<code>return</code>方法必须返回一个对象，这是 Generator 规格决定的。</p> <p><code>throw</code>方法主要是配合 Generator 函数使用，一般的遍历器对象用不到这个方法。请参阅《Generator 函数》一章。</p> <h2 id="for-of-循环"><a href="#for-of-循环" class="header-anchor">#</a> for...of 循环</h2> <p>ES6 借鉴 C++、Java、C# 和 Python 语言，引入了<code>for...of</code>循环，<strong>作为遍历所有数据结构的统一的方法</strong>。</p> <p>一个数据结构只要部署了<code>Symbol.iterator</code>属性，就被视为具有 iterator 接口，就可以用<code>for...of</code>循环遍历它的成员。也就是说，<strong><code>for...of</code>循环内部调用的是数据结构的<code>Symbol.iterator</code>方法</strong>。</p> <p><code>for...of</code>循环可以使用的范围包括<strong>数组、Set 和 Map 结构、某些类似数组的对象（比如<code>arguments</code>对象、DOM NodeList 对象）、后文的 Generator 对象，以及字符串。</strong></p> <h3 id="数组"><a href="#数组" class="header-anchor">#</a> 数组</h3> <p>数组原生具备<code>iterator</code>接口（即默认部署了<code>Symbol.iterator</code>属性），<code>for...of</code>循环本质上就是调用这个接口产生的遍历器，可以用下面的代码证明。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> arr <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token string">'red'</span><span class="token punctuation">,</span> <span class="token string">'green'</span><span class="token punctuation">,</span> <span class="token string">'blue'</span><span class="token punctuation">]</span><span class="token punctuation">;</span>

<span class="token keyword">for</span><span class="token punctuation">(</span><span class="token keyword">let</span> v <span class="token keyword">of</span> arr<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>v<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// red green blue</span>
<span class="token punctuation">}</span>

<span class="token keyword">const</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
obj<span class="token punctuation">[</span>Symbol<span class="token punctuation">.</span>iterator<span class="token punctuation">]</span> <span class="token operator">=</span> arr<span class="token punctuation">[</span>Symbol<span class="token punctuation">.</span>iterator<span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">bind</span><span class="token punctuation">(</span>arr<span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token keyword">for</span><span class="token punctuation">(</span><span class="token keyword">let</span> v <span class="token keyword">of</span> obj<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>v<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// red green blue</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br></div></div><p>上面代码中，空对象<code>obj</code>部署了数组<code>arr</code>的<code>Symbol.iterator</code>属性，结果<code>obj</code>的<code>for...of</code>循环，产生了与<code>arr</code>完全一样的结果。</p> <p><code>for...of</code>循环可以代替数组实例的<code>forEach</code>方法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> arr <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token string">'red'</span><span class="token punctuation">,</span> <span class="token string">'green'</span><span class="token punctuation">,</span> <span class="token string">'blue'</span><span class="token punctuation">]</span><span class="token punctuation">;</span>

arr<span class="token punctuation">.</span><span class="token function">forEach</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">element<span class="token punctuation">,</span> index</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>element<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// red green blue</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>index<span class="token punctuation">)</span><span class="token punctuation">;</span>   <span class="token comment">// 0 1 2</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>JavaScript 原有的**<code>for...in</code>循环，只能获得对象的键名**，不能直接获取键值。ES6 提供<code>for...of</code>循环，允许遍历获得键值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> arr <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">,</span> <span class="token string">'c'</span><span class="token punctuation">,</span> <span class="token string">'d'</span><span class="token punctuation">]</span><span class="token punctuation">;</span>

<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> a <span class="token keyword">in</span> arr<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>a<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 0 1 2 3</span>
<span class="token punctuation">}</span>

<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> a <span class="token keyword">of</span> arr<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>a<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// a b c d</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p>上面代码表明，<strong><code>for...in</code>循环读取键名，<code>for...of</code>循环读取键值</strong>。如果要通过<code>for...of</code>循环，获取数组的索引，可以借助数组实例的<code>entries</code>方法和<code>keys</code>方法（参见《数组的扩展》一章）。</p> <p><code>for...of</code>循环调用遍历器接口，数组的遍历器接口只返回具有数字索引的属性。这一点跟<code>for...in</code>循环也不一样。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> arr <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">5</span><span class="token punctuation">,</span> <span class="token number">7</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
arr<span class="token punctuation">.</span>foo <span class="token operator">=</span> <span class="token string">'hello'</span><span class="token punctuation">;</span>

<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> i <span class="token keyword">in</span> arr<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// &quot;0&quot;, &quot;1&quot;, &quot;2&quot;, &quot;foo&quot;</span>
<span class="token punctuation">}</span>

<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> i <span class="token keyword">of</span> arr<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">//  &quot;3&quot;, &quot;5&quot;, &quot;7&quot;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>上面代码中，<code>for...of</code>循环不会返回数组<code>arr</code>的<code>foo</code>属性。</p> <h3 id="set-和-map-结构"><a href="#set-和-map-结构" class="header-anchor">#</a> Set 和 Map 结构</h3> <p>Set 和 Map 结构也原生具有 Iterator 接口，可以直接使用<code>for...of</code>循环。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> engines <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Set</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token string">&quot;Gecko&quot;</span><span class="token punctuation">,</span> <span class="token string">&quot;Trident&quot;</span><span class="token punctuation">,</span> <span class="token string">&quot;Webkit&quot;</span><span class="token punctuation">,</span> <span class="token string">&quot;Webkit&quot;</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">var</span> e <span class="token keyword">of</span> engines<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>e<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">// Gecko</span>
<span class="token comment">// Trident</span>
<span class="token comment">// Webkit</span>

<span class="token keyword">var</span> es6 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Map</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
es6<span class="token punctuation">.</span><span class="token function">set</span><span class="token punctuation">(</span><span class="token string">&quot;edition&quot;</span><span class="token punctuation">,</span> <span class="token number">6</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
es6<span class="token punctuation">.</span><span class="token function">set</span><span class="token punctuation">(</span><span class="token string">&quot;committee&quot;</span><span class="token punctuation">,</span> <span class="token string">&quot;TC39&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
es6<span class="token punctuation">.</span><span class="token function">set</span><span class="token punctuation">(</span><span class="token string">&quot;standard&quot;</span><span class="token punctuation">,</span> <span class="token string">&quot;ECMA-262&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">var</span> <span class="token punctuation">[</span>name<span class="token punctuation">,</span> value<span class="token punctuation">]</span> <span class="token keyword">of</span> es6<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>name <span class="token operator">+</span> <span class="token string">&quot;: &quot;</span> <span class="token operator">+</span> value<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">// edition: 6</span>
<span class="token comment">// committee: TC39</span>
<span class="token comment">// standard: ECMA-262</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br></div></div><p>上面代码演示了如何遍历 Set 结构和 Map 结构。值得注意的地方有两个，首先，遍历的顺序是按照各个成员被添加进数据结构的顺序。其次，Set 结构遍历时，返回的是一个值，而 Map 结构遍历时，返回的是一个数组，该数组的两个成员分别为当前 Map 成员的键名和键值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> map <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Map</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">set</span><span class="token punctuation">(</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">set</span><span class="token punctuation">(</span><span class="token string">'b'</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> pair <span class="token keyword">of</span> map<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>pair<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">// ['a', 1]</span>
<span class="token comment">// ['b', 2]</span>

<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> <span class="token punctuation">[</span>key<span class="token punctuation">,</span> value<span class="token punctuation">]</span> <span class="token keyword">of</span> map<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>key <span class="token operator">+</span> <span class="token string">' : '</span> <span class="token operator">+</span> value<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">// a : 1</span>
<span class="token comment">// b : 2</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br></div></div><h3 id="计算生成的数据结构"><a href="#计算生成的数据结构" class="header-anchor">#</a> 计算生成的数据结构</h3> <p>有些数据结构是在现有数据结构的基础上，计算生成的。比如，ES6 的数组、Set、Map 都部署了以下三个方法，调用后都返回遍历器对象。</p> <ul><li><code>entries()</code> 返回一个遍历器对象，用来遍历<code>[键名, 键值]</code>组成的数组。对于数组，键名就是索引值；对于 Set，键名与键值相同。Map 结构的 Iterator 接口，默认就是调用<code>entries</code>方法。</li> <li><code>keys()</code> 返回一个遍历器对象，用来遍历所有的键名。</li> <li><code>values()</code> 返回一个遍历器对象，用来遍历所有的键值。</li></ul> <p>这三个方法调用后生成的遍历器对象，所遍历的都是计算生成的数据结构。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> arr <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">,</span> <span class="token string">'c'</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> pair <span class="token keyword">of</span> arr<span class="token punctuation">.</span><span class="token function">entries</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>pair<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">// [0, 'a']</span>
<span class="token comment">// [1, 'b']</span>
<span class="token comment">// [2, 'c']</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><h3 id="类似数组的对象"><a href="#类似数组的对象" class="header-anchor">#</a> 类似数组的对象</h3> <p>类似数组的对象包括好几类。下面是<code>for...of</code>循环用于字符串、DOM NodeList 对象、<code>arguments</code>对象的例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 字符串</span>
<span class="token keyword">let</span> str <span class="token operator">=</span> <span class="token string">&quot;hello&quot;</span><span class="token punctuation">;</span>

<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> s <span class="token keyword">of</span> str<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>s<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// h e l l o</span>
<span class="token punctuation">}</span>

<span class="token comment">// DOM NodeList对象</span>
<span class="token keyword">let</span> paras <span class="token operator">=</span> document<span class="token punctuation">.</span><span class="token function">querySelectorAll</span><span class="token punctuation">(</span><span class="token string">&quot;p&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> p <span class="token keyword">of</span> paras<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  p<span class="token punctuation">.</span>classList<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token string">&quot;test&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token comment">// arguments对象</span>
<span class="token keyword">function</span> <span class="token function">printArgs</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> x <span class="token keyword">of</span> arguments<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token function">printArgs</span><span class="token punctuation">(</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// 'a'</span>
<span class="token comment">// 'b'</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br></div></div><p>对于字符串来说，<code>for...of</code>循环还有一个特点，就是会正确识别 32 位 UTF-16 字符。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> x <span class="token keyword">of</span> <span class="token string">'a\uD83D\uDC0A'</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">// 'a'</span>
<span class="token comment">// '\uD83D\uDC0A'</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>并不是所有类似数组的对象都具有 Iterator 接口，一个简便的解决方法，就是使用<code>Array.from</code>方法将其转为数组。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> arrayLike <span class="token operator">=</span> <span class="token punctuation">{</span> length<span class="token operator">:</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token operator">:</span> <span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token operator">:</span> <span class="token string">'b'</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token comment">// 报错</span>
<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> x <span class="token keyword">of</span> arrayLike<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token comment">// 正确</span>
<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> x <span class="token keyword">of</span> Array<span class="token punctuation">.</span><span class="token function">from</span><span class="token punctuation">(</span>arrayLike<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><h3 id="对象"><a href="#对象" class="header-anchor">#</a> 对象</h3> <p>对于普通的对象，<code>for...of</code>结构不能直接使用，会报错，必须部署了 Iterator 接口后才能使用。但是，这样情况下，<code>for...in</code>循环依然可以用来遍历键名。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> es6 <span class="token operator">=</span> <span class="token punctuation">{</span>
  edition<span class="token operator">:</span> <span class="token number">6</span><span class="token punctuation">,</span>
  committee<span class="token operator">:</span> <span class="token string">&quot;TC39&quot;</span><span class="token punctuation">,</span>
  standard<span class="token operator">:</span> <span class="token string">&quot;ECMA-262&quot;</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> e <span class="token keyword">in</span> es6<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>e<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">// edition</span>
<span class="token comment">// committee</span>
<span class="token comment">// standard</span>

<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> e <span class="token keyword">of</span> es6<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>e<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">// TypeError: es6[Symbol.iterator] is not a function</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br></div></div><p>上面代码表示，对于普通的对象，<code>for...in</code>循环可以遍历键名，<code>for...of</code>循环会报错。</p> <p>一种解决方法是，使用<code>Object.keys</code>方法将对象的键名生成一个数组，然后遍历这个数组。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">var</span> key <span class="token keyword">of</span> Object<span class="token punctuation">.</span><span class="token function">keys</span><span class="token punctuation">(</span>someObject<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>key <span class="token operator">+</span> <span class="token string">': '</span> <span class="token operator">+</span> someObject<span class="token punctuation">[</span>key<span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>另一个方法是使用 Generator 函数将对象重新包装一下。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">entries</span><span class="token punctuation">(</span><span class="token parameter">obj</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> key <span class="token keyword">of</span> Object<span class="token punctuation">.</span><span class="token function">keys</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">yield</span> <span class="token punctuation">[</span>key<span class="token punctuation">,</span> obj<span class="token punctuation">[</span>key<span class="token punctuation">]</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>

<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> <span class="token punctuation">[</span>key<span class="token punctuation">,</span> value<span class="token punctuation">]</span> <span class="token keyword">of</span> <span class="token function">entries</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>key<span class="token punctuation">,</span> <span class="token string">'-&gt;'</span><span class="token punctuation">,</span> value<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">// a -&gt; 1</span>
<span class="token comment">// b -&gt; 2</span>
<span class="token comment">// c -&gt; 3</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br></div></div><h3 id="与其他遍历语法的比较"><a href="#与其他遍历语法的比较" class="header-anchor">#</a> 与其他遍历语法的比较</h3> <p>以数组为例，JavaScript 提供多种遍历语法。最原始的写法就是<code>for</code>循环。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">var</span> index <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> index <span class="token operator">&lt;</span> myArray<span class="token punctuation">.</span>length<span class="token punctuation">;</span> index<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>myArray<span class="token punctuation">[</span>index<span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>这种写法比较麻烦，因此数组提供内置的<code>forEach</code>方法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>myArray<span class="token punctuation">.</span><span class="token function">forEach</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">value</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>这种写法的问题在于，无法中途跳出<code>forEach</code>循环，<code>break</code>命令或<code>return</code>命令都不能奏效。</p> <p><code>for...in</code>循环可以遍历数组的键名。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">var</span> index <span class="token keyword">in</span> myArray<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>myArray<span class="token punctuation">[</span>index<span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p><code>for...in</code>循环有几个缺点。</p> <ul><li>数组的键名是数字，但是<code>for...in</code>循环是以字符串作为键名“0”、“1”、“2”等等。</li> <li><code>for...in</code>循环不仅遍历数字键名，还会遍历手动添加的其他键，甚至包括原型链上的键。</li> <li>某些情况下，<code>for...in</code>循环会以任意顺序遍历键名。</li></ul> <p>总之，<code>for...in</code>循环主要是为遍历对象而设计的，不适用于遍历数组。</p> <p><code>for...of</code>循环相比上面几种做法，有一些显著的优点。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> value <span class="token keyword">of</span> myArray<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><ul><li>有着同<code>for...in</code>一样的简洁语法，但是没有<code>for...in</code>那些缺点。</li> <li>不同于<code>forEach</code>方法，它可以与<code>break</code>、<code>continue</code>和<code>return</code>配合使用。</li> <li>提供了遍历所有数据结构的统一操作接口。</li></ul> <p>下面是一个使用 break 语句，跳出<code>for...of</code>循环的例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">var</span> n <span class="token keyword">of</span> fibonacci<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">if</span> <span class="token punctuation">(</span>n <span class="token operator">&gt;</span> <span class="token number">1000</span><span class="token punctuation">)</span>
    <span class="token keyword">break</span><span class="token punctuation">;</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>n<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面的例子，会输出斐波纳契数列小于等于 1000 的项。如果当前项大于 1000，就会使用<code>break</code>语句跳出<code>for...of</code>循环。</p></div></div> <div class="page-slot page-slot-bottom"><!-- 横向自适应 -->
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